College of Resources and Environment, Sichuan Agricultural University, Chengdu 611130, P. R. China;
State Key Laboratory of Earth Surface Processes and Resource Ecology, Beijing Normal University,Beijing 100875, P. R. China;
State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Water and Soil Conservation, Chinese Academy of Sciences and Ministry of Water Resources,Yangling 712100, P. R. China;
College of Resources and Environment, Sichuan Agricultural University, Chengdu 611130, P. R. China;
College of Resources and Environment, Northwest A &F University, Yangling 712100, P. R. China;